1. Field of the Invention
This invention relates to a process for purifying used lubricating oils. More particularly this invention relates to reclaiming used lubricating oil to produce as major products a lubricating oil base stock suitable for blending and an environmentally safe solid, coked residue suitable for landfilling.
2. Description of the Prior Art
Virgin lubricating oils are derived typically from waxy petroleum distillate oil stocks. Such waxy petroleum distillate oil stocks have a viscosity of less than 50 SUS at 100.degree. F. and have a boiling range of about 600.degree. F. to 650.degree. F. (315.degree. C. to 343.degree. C.) initial boiling point to about 1050.degree. F. to 1100.degree. F. (566.degree. C. to 593.degree. C.) end point. Such waxy petroleum distillate oil stocks may be derived from raw lube oil stocks the major portion of which boil above 650.degree. F. (343.degree. C.). These raw lube stocks are vacuum distilled with overhead and side draw distillate streams and a bottom stream referred to a residual oil stock. Considerable overlap in boiling ranges of distillate streams and the residual stream may exist, depending upon distillation efficiency. Some heavier distillates have almost the same distribution of molecular species as the residual stream. Both paraffinic and naphthenic crude oils are used as sources of lube oil stocks with paraffinic crudes giving the best yields of high viscosity index product, hence these are preferred for most lubricant applications.
Such distillate streams contain aromatic and polar compounds which are undesirable in lubricating oils. Such compounds are removed by means such as solvent extraction, hydrogenation and other means well known in the art, either before or after solvent dewaxing.
The wax content of a waxy distillate oil stock is defined by the amount of material to be removed to produce a dewaxed oil with a selected pour point temperature in the range of about +25.degree. F. to -40.degree. F. (-3.9.degree. C. to -40.degree. C.). Wax content of waxy distillate oil stock will vary in the range of 5 wt % to 35 wt %. Distillate oil stock is dewaxed typically by solvent dewaxing, however catalytic dewaxing processes have been found which will become industrially significant.
The dewaxed product is referred to as a lubricating oil base stock and is suitable for blending with other base stocks to achieve various desired properties. The blended base stock is then combined with additives such as soaps, E. P. agents, VI improvers and polymeric dispersants to produce an engine lubricating oil of SAE 5 to SAE 60.
After use, this oil is collected from truck and bus fleets and automobile service stations for reclaiming. Ideally this collected oil is used oil of grade SAE 5 to SAE 60 and will ordinarily contain metal containing compounds and sludge formed in the engine. However, typically when the collection is not supervised by the processor the used oil will typically contain waste grease, edible fats and oils, water and waste of unknown origin referred to broadly as undesirable components. Used lubricating oil can contain all of these components and mixtures thereof.
Crude oils which are high in distillates of the lubricating oil range are in considerable demand. They correspondingly demand a premium price. This has made reclaiming of used lubricating oils an increasingly interesting option for satisfying lubricating oil demand. It would seem that a single, best process for reclaiming used lubricating oil should have been developed which produces a reclaimed lubricating oil of acceptable quality. This is not the case. At its present state of development waste oil reclaiming is a distribution of small processors each one of which uses a different process, responsive to the waste oil available, product demand and environmental considerations in the geographic area. These different processes include as the key processing steps: chemical demetallization, specialized distillation, solvent extraction, acid treating and clay treating. These processes all suffer from a common defect, i.e., difficulty in disposing of the final metal-containing residuum in an environmentally acceptable manner.
There exists a need in the art for a lubricating oil reclaiming process which produces an oil of good quality while producing a waste stream which is environmentally acceptable, such as in a landfill.